A ninth chord is formed by adding a ninth to a seventh chord.

The most common ninth chords are built over the dominant seventh chord in both major and minor keys. Because the ninth formed is major in major keys and minor in minor keys, these chords are called dominant major ninth and dominant minor ninth.

Ninth chords can also be built on minor seventh and major seventh chords:

The augmented or sharp ninth chord is obtained by adding an augmented ninth interval to a dominant seventh chord. In many cases, the ninth interval is simplified enharmonically. In these cases, the chord appears as a dominant ninth chord with a major and minor third:

We find an augmented ninth chord in Chopin's Prelude 4:

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